The invention relates to a fuel supply system for an internal combustion engine for motor vehicles such as trucks, passenger cars, and buses, which include a fuel tank for liquid fuel from which fuel is supplied to a fuel processing apparatus by which the fuel is separated into fractions which are then supplied to the different cylinders of the engine depending on the operating conditions of the engine.
The fuels available presently for the operation of internal combustion engines of motor vehicles are a compromise between limits established by the fuel manufacturers, for example because of different crude oil qualities, processing equipment, costs and energy requirements and which are, on the other hand, partially requirements contrary to those of the internal combustion engine, based on cold start capability even at extremely low temperatures, low exhaust gas and evaporation emissions, low fuel consumption, high knock resistance also in supercharged engines, low or no corrosion, low sulfur content, smooth engine operation and high safety.
In order to be able to adapt the available fuels better to the requirements of the internal combustion engine, fuel supply and preparation apparatus are provided.
U.S. Pat. No. 4,220,120 discloses a fuel supply system with a fuel processing apparatus, including a fuel tank for liquid fuel, a fuel processing device to which fuel from the tank is supplied and which produces at least two different fuel parts for admission to the engine. Control means are provided which make it possible to supply the respective fuel parts and the fuel separately to the internal combustion engine depending on certain operating conditions of the engine. The control means include a mixing structure to which the fuel or the fuel parts are first supplied and which produces therefrom a mixture which is dependent on the operating conditions of the engine and which is then supplied to the internal combustion engine.
DE 197 34 493 C1 discloses another fuel supply system with a fuel tank for liquid fuel. It further includes a fuel processing apparatus, that is, an evaporation and condensation structure to which the fuel from the fuel tank is supplied and which generates therefrom a highly volatile fuel fraction and a fuel fraction of lower volatility and which supplies the two fuel components to the engine. The high volatility fuel fraction is stored in an intermediate tank 16 and the low volatility fuel fraction is stored in an auxiliary tank 21. Furthermore, the known fuel supply system includes a main fuel pump 5 whose suction side is connected, by way of a first switch-over valve 6 either to the fuel tank containing the fuel or to the additional tank containing the low volatility fuel and whose pressure side is connected to a first inlet of a second switch-over valve 3. A second inlet of this second switchover valve 3 is connected to the pressure side of an auxiliary fuel pump 17 whose inlet is connected to the intermediate tank containing the high volatility fuel fraction. The outlet side of this switchover valve 3 leads to the internal combustion engine. With the different fuel pumps, the first switchover valve and the second switchover valve and a corresponding control arrangement, this known fuel system can supply the respective fuel parts separately to the engine depending on certain engine operating conditions. For the start up of the internal combustion engine and during a warm-up phase, the auxiliary fuel pump 17 is operated so that, by way of the second switchover valve 3, exclusively high volatile fuel is supplied to the engine. Under full load engine operation, when the main fuel pump 7 is operated, the switchover valve 6 is so actuated that the low volatile fuel is supplied from the additional tank 21 to the engine. Under all intermediate operating conditions of the internal combustion engine, the switch-over valve 6 is so controlled that the fuel from the fuel tank is supplied by the main fuel pump to the engine.
The present invention is concerned with the problem of improving the capability of such a fuel supply system to adapt the available fuel to the requirements of the internal combustion engine.
In a fuel supply system for an internal combustion engine including a fuel tank, a fuel processing apparatus to which fuel from the tank is supplied and which separates the fuel into different fractions, the fuel and the fractions are mixed in a mixing chamber according to a performance graph stored in a control unit depending on the operating state of the engine and the mixture is then supplied to the engine in a controlled manner.
The invention is based on the general concept of separating the fuel supplied from the fuel tank by means of a fuel processing system into fuel fractions, that is, into the individual components or component mixtures of the fuel in the tank and to form from these fractions, in a mixing apparatus, a fuel mixture which is optimal for the momentary operating state of the internal combustion engine. The fuel mixture to be supplied to the engine is provided by controlled mixing of the previously separated fuel fractions with each other or with fuel from the fuel tank.
For this purpose, the mixing apparatus preferably includes a suitable control unit, which determines from a performance graph the optimal fuel mixture for the momentary operating state of the engine and adjusts the mixing apparatus so as to provide the optimum mixture. The optimal mixture for the various engine-operating conditions can be determined, for example, by field-testing and the values can be stored so as to be available for the adjustment of the mixture. The engine operating data for determining the momentary engine operating state are available to the mixing apparatus control unit from the electronic engine operating control unit so that no special means are necessary to collect the respective engine operating data.
In a preferred embodiment, the fuel fractions produced by the fuel processing system may differ by their boiling temperatures and/or their sulfur content and/or their cetane number and/or their reduction medium content. A fuel fraction of high volatility, that is one with a low boiling temperature, results with gasoline fuels for example in a large reduction of the HC emissions of the internal combustion engine. In contrast, a fuel fraction with a higher boiling temperature, which has a higher octane number, permits an increase of the power output of the engine and, at the same time, provides for lower fuel consumption. With a fuel, which has a low boiling point and a low sulfur content, poisoning of the catalytic converter can be avoided, particularly with direct fuel injection engine using the stratified charge principle. But an engine using a fuel fraction with a high boiling point and a high sulfur content and high octane number operates at or under full load essentially stoichiometrically. In connection with Diesel fuel, a fuel fraction with a reduced cetane number may be provided during normal operation of the internal combustion engine; for cold start and during warm-up, a fuel fraction with a higher Cetane number is advantageous. Furthermore, a fuel fraction with an increased reduction medium content may improve the engine emission values particularly for the after-treatment of the exhaust gases when the engine is operated with a lean mixture.
Further features and advantages of the fuel system according to the invention will become apparent from the following description of the invention on the basis of the accompanying drawings.